Patent GB-894689-A (GB)
Title: Speed control circuits for electric motors
IPC: H02P7/26
Filed: 1960-01-05
Abstract: 894,689. Control of two motors. MINNESOTA MINING & MANUFACTURING CO. Jan. 5, 1960 [Jan. 8, 1959], No. 390/60. Class 38 (3). [Also in Group XXXV] A D.C. motor 9 drives capstan 14 forming part of a magnetic tape recording and reproducing equipment, the speed of the motor being set by stepping switches 21, 22, so that the resonant frequency of a circuit comprising inductor 29 and one of the capacitors 23- 28 determines the motor speed. The motor 9 and a take-up motor 19 are started by closing switch 12 to energize relay 13 for completing the circuit of armature 10 from voltagedoubler circuit 30 through triodes 31, 32. A tachometer generator 40 is connected to valve 44 providing at its anode and cathode two sinusoidal signals respectively displaced 180 degrees in phase from each other and having frequencies equal to the frequency developed by the tachometer generator. The two signals from the valve 44 are supplied to a phase-changing network 60, a signal derived from capacitor 64 leading the signal from resistor 62 by 90 degrees. These signals are amplified in valves 70a, 70b, the signal from valve 70b being applied to the grid of valve 81b without further phaseshift, whilst the lagging signal from the valve 70a is adapted to be further phase-shifted by the tuned circuit comprising inductor 29 and capacitor 28 before the signal is applied to the grid of the valve 81a. When the frequency of the signals from the valve 70a is less than or greater than the resonant frequency, a leading or lagging phase-shift is provided by the tuned circuit. The signals from valves 81a, 816 are applied respectively to the control and suppressor grids of pentode 93 which passes current only when both input signals are relatively positive, the pentode operating to convert the sinusoidal signals from the valves 81a, 81b, into signals having rectangular characteristics. When the two signals supplied to the pentode 93 are in phase, the pulse duty factor is one-half, whereas, if the two signals are 180 degrees out of phase, the factor is zero. A triode 98 inverts the signals from tube 93 and its output is filtered to produce a direct potential which varies in magnitude as the frequency of the developed signal changes in relation to the resonant frequency. Direct voltage provided by diodes 129, 130, opposes the voltage from valve 98, a rheostat 105 adjusting the voltage applied to the grids of tubes 31, 32. Thus, as the potential on the grids decreases due to an increase in frequency of the developed signal, the current through the armature 10 decreases. When the switch 12 is closed, the valve 816 transmits a positive control signal to valve 90 for operating relay 17 to energize the take-up motor 19. When the motor 9 reaches its operating speed, an over-riding signal from valve 81a causes the valve 90 to become non-conductive so that relay 17 is deenergized. When the relay 13 is de-energized by opening switch 15, the armature 10 is disconnected from its supply circuit and is shunted by braking resistor 160. As the motor decelerates, the over-riding signal from the valve 81a decreases and the valve 90 becomes conductive for a short period to re-operate relay 17. This prevents the starting relay 13 from being energized until the motor 9 reaches standstill.